Due to heat generated in the operation of gas turbine engines, various methods and system have been developed for rejecting heat of engine systems such as the auxiliary gearbox, the integrated drive generator, etc. Heat may be rejected to available cooling air, such as air circulating in the bypass duct. Accordingly, heat exchangers may project into the bypass duct, but may hence affect air flow and cause vibration of the fan. It is also known to position coils of cooling oil near or at the surface of the bypass duct, bringing flammable fluids near the stream of bypass air.